1. Field of the Invention
The present invention relates to a high-voltage semiconductor device using silicon carbide (SiC).
2. Description of the Related Art
SiC is expected to be the material of power semiconductor devices of the next generation. SiC exhibits excellent physical properties. Its band gap is three times as large as that of Si, its breakdown field strength is about ten times as high as that of Si, and its thermal conductivity is approximately three times as high as that of Si. Having such desirable physical properties, SiC can help to provide power semiconductor devices that operate at a very small loss and at high temperatures.
Various high-voltage semiconductor devices are available, which utilize the physical properties of SiC. Known as such a semiconductor device is double implantation MOSFET (hereinafter referred to as DIMOSFET (see, for example, R. Kosugi et al., Materials Science Forum, Vols. 457-460 (2004), pp. 1397-1400). The DIMOSFET is a device easy to manufacture because its channel can be formed by ion implantation in the planer process. Further, the DIMOSFET is can be driven by a small-power drive circuit, because its gate is voltage-controlled. In view of this, the DIMOSFET is a desirable device fit to parallel operation.
However, the self-align process performed by means of thermal diffusion in manufacturing Si devices cannot be employed in order to manufacture SiC devices. Therefore, to form the channel region of a SiCMOSFET, the base and source regions are independently formed by ion implantation, using two masks. The resistance in the channel region must be lowered in order to decrease the on-resistance of MOSFET. To this end, the channel length should be reduced to about 0.5 μm. Here arises a problem. If a channel 0.5 μm long is formed by ion implantation using two masks, the channel resistance will change by 20% from the design value even if the alignment error is only 0.1 μm. This hinders the miniaturization of SiCMOSFETs.
As described above, the base region and source region of the conventional DIMOSFET are independently formed by ion implantation that uses two masks. The channel length is determined by the distance between an edge of the base region and that edge of the source region which opposes that edge of the base region. Inevitably, the channel length deviates from the design value due to an error made in aligning the ion-implantation masks. Consequently, the device will have an on-resistance that differs from the design value. This hinders the miniaturization of the device.
Accordingly, it is demanded that a SiC semiconductor device of a very low on-resistance, which can be manufactured without errors in aligning the ion-implantation masks and can therefore has a short channel, is realized.